1. Field of the Invention
This invention relates to a control system for a mechanical press, and, more particularly, to an automatic control system for adjusting the counterbalance for such a press.
2. Description of the Prior Art
Mechanical presses are commonly used for stamping, bending, blanking, embossing and otherwise shaping materials, usually metals. Replaceable die sets are used to do such forming with a lower die half attached to a stationery bed or platen and an upper die half attached to a ram or slide which reciprocates vertically. Typically in a mechanical press, an electric motor is used to rotate a counter weight, bringing the rotational speed of the counter weight up to a no-load equilibrium speed which will provide the proper kinetic energy to perform the forming operation. A clutch mechanism engages the flywheel which, through gears and mechanical linkage, including at least one crank, reciprocates the ram in its working cycle using the inertia of the flywheel. Counterbalances are used to counterbalance the moving weight of the ram and its attached upper die half or punch to provide smooth operation, easier stopping, and less wear on the gears, bearings and other moving parts of the press.
Mechanical presses commonly use one or more pneumatic cylinders to perform the counterbalancing function. Typically, the air pressure is adjusted by the press operator through a manual pressure regulator when a new set of dies are put into the press to compensate for the change in the weight of the die. Usually, very little further adjustments are made of the air pressure, unless they are made pursuant to the "feel" that an experienced operator has in the efficient running of the press.
Systems have been developed for automatically adjusting the air pressure in an attempt to compensate for various effects. For example, in U.S. Pat. No. 4,283,929, the die sets, or at least the upper die or punch member is encoded so that when a new die set is put into the machine, this coding is read by the machine to automatically make an adjustment in the counterbalance air pressure to compensate for the change in the weight of the die. Other attempts have been made to automatically compensate for change in the die weight by measurement of the motor current only. U.S. Pat. No. 4,069,697 teaches changing the air pressure responsive to a current signal so that adjustments for excess counterbalancing is accomplished on a down stroke and compensation for insufficient counterbalancing is done on an upstroke. Unfortunately, adjustments for die weight or motor current only solves part of the problem.
In our U.S. Pat. No. 4,969,344 we have set forth a control system for automatically counterbalancing the ram of a mechanical press by measuring the energy level of the press flywheel and varying air pressure in one or more counterbalancing cylinders. The energy level is derived from a measurement of the rotational speed of the flywheel at a fixed point in the ram reciprocation cycle. The energy level of the flywheel in an alternative embodiment is determined by measuring the power drawn by the electric motor which in turn engages the flywheel. It should be noted that in this mode the total power must be measured and not merely the motor current which can vary with line voltage and power factor.
In our U.S. Pat. No. 5,009,091 we set forth another control system for automatically counterbalancing the ram by varying air pressure in one or more counterbalancing cylinders with the energy level of the press flywheel being determined by a measurement of the linear speed of a moving press component at a fixed point in the ram reciprocation cycle. The speed of the ram can be determined by measuring the speed of one or more of the press drive components. In a more complex embodiment of this control system, a first sensor detects the speed of the ram during an upstroke, a second sensor detects the speed of the ram during a downstroke and another sensor measures the rotational speed of the flywheel. All of these measurements are used by the control means to determine if a counterbalance condition needs to be changed by increasing or decreasing the pressure in the counterbalance air cylinders.